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Here, we demonstrate a continuous, wearable and wireless vital-sign monitor worn at the ear in the form factor of a hearing aid. The ear location is chosen because it is a natural anchoring point, and it is discreet since the device can be partially hidden by hair and the ear. It is also chosen so that all physiological signals can be acquired from one location for improved patient compliance. The monitor measures single lead ECG, BCG, and PPG. These physiological signals allow the monitor to measure heart rate from three independent sources, and blood oxygenation. Additionally, information embedded in the peak timings between the signals has been shown to correlate to the preejection period (PEP), heart contractility, stroke volume, cardiac output and mean arterial blood pressure, the last of which will be discussed in this paper. The head PPG is taken in reflectance mode, with light reflecting off the mastoid bone to photodiodes in the same plane as the LED light sources. The PPG has a small pulsatile component, usually between 0.25 – 1% of a large static component. This large static current presents many challenges for low-voltage, single-supply systems. Typically, reducing the LED drive current can maintain the output of the transimpedance amplifier (TIA) within the supply rails, at the expense of lower signal-to-noise ratio (SNR).
![bias amp 2 ptt bias amp 2 ptt](https://i.ytimg.com/vi/jPSIQDyKm90/maxresdefault.jpg)
Alternatively, reducing the transimpedance of the feedback element will achieve the same result. developed a static photocurrent subtraction circuit by using an error amplifier in feedback with the transimpedance amplifier to drive a current source into the cathode of the photo-diodes. However, the frequency response of the TIA changes due to the amount of static photocurrent subtracted, and can reduce the bandwidth to unacceptably low levels. Tavakoli utilized a log-amp in the front end to increase dynamic range, requiring a full signal path for each PPG wavelength.
![bias amp 2 ptt bias amp 2 ptt](https://i.stack.imgur.com/ow8UH.png)
To address these concerns, a new photoreceptor circuit has been designed, in which the photodiodes are forward biased ( Fig. A digital-to-analog converter (DAC) is used to generate a reference voltage V ref at the positive terminal of the TIA.